The present invention relates to a method of producing an electrical conductor suitable for transmitting multi-frequency signal such as an audio signal or a video signal.
As is well known, an electrical conductor is generally made of one of two types of copper: tough pitch copper (TPC) and oxygen-free copper (OFC) The copper is generally worked so as to be circular or rectangular in cross section or in the form of a foil and then recrystallized by annealing at a temperature ranging between 300.degree. and 600.degree. C. to produce the conductor.
It has recently been understood that an electrical conductor made of OFC in a transmission line for a multi-frequency audio signal, particularly, as an inner wiring conductor or a loud speaker wiring conductor of an audio apparatus, is much superior to one made of TPC. The reason for this has been considered that the conductivity of OFC is higher than that of TPC. Based on this concept, it was thought that superior sound signals could be obtained by increasing the cross-sectional area of a conductor. However, it has been revealed by the experiments conducted by the inventor that the sound signal quality does not change very much when the cross-sectional area of the conductor is changed.
These precious findings, as described hereinafter, were obtained by the inventor's study of the relationship between the tone quality of an audio apparatus and an electrical conductor used therein.
Metallic copper is composed of a number of fine crystals in the ordinary state, and oxygen contained in the copper is present along crystal boundaries, mainly in the form of cuprous oxide, which is inherently a semiconductor. Thus, the presence of such cuprous oxide means that distributed capacitances are associated with each grain; both a parallel capacitance across the grain and a series capacitance between adjacent grains.
The dielectric effect of the cuprous oxide along crystal boundaries increases the magnitude of the distributed capacitances. Also, the magnitudes of the distributed capacitances changes depending on the frequency. Hence, an audio signal including high-frequency components is subject to distortion in phase and an attenuation in amplitude due to this inherent capacitance. Such distortion in phase and attenuation is detectable by the ear.
In an electrical conductor of OFC produced by annealing at a temperature of about 400.degree. C., the average grain size of crystals is about 20 .mu.m. Thus, it may be considered that about 50,000 capacitors per meter are present in the lengthwise direction of such a conductor. On the other hand, in an electrical conductor made of TPC, about 150,000 capacitors per meter are present in the conductor since the crystal grain diameter of TPC is about one-third to one-fourth that of OFC.
In view of the above described fact, the use of an OFC conductor results in a superior sound quality to that obtained with TPC. This finding applies not only to audio signals but also to video and other signals.
An electrical conductor of OFC is much superior to a conductor of TPC with respect to transmission of multi-frequency signal components. However, an OFC conductor still contains, as described above, a large amount of cuprous oxide along crystal grain boundaries and, therefore it is of insufficient quality to realize optimum transmission characteristics.